In recent years, polarizing plates, phase difference plates, and the like have been actively developed using polymerizable liquid crystal materials. They are obtained by coating a base material subjected to a rubbing treatment with a solution containing a polymerizable liquid crystal material, drying a solvent and, thereafter, causing polymerization by ultraviolet rays or heat. Meanwhile, it is also known that, when a polymerizable cholesteric liquid crystal material is prepared by adding a chiral compound to a polymerizable liquid crystal material, a circularly polarized light separation element is obtained in the same manner and, for example, applications to brightness enhancement films and the like have been studied.
Various thin films produced by polymerizing these polymerizable liquid crystal materials are required to have an appropriate phase difference and the uniformity thereof and, furthermore, low haze, the heat resistance, the mechanical strength, the adhesion to base material, and the like. In addition, the brightness enhancement film by using the polymerizable cholesteric liquid crystal material is also required to have high bandwidth reflection wavelength characteristics and high reflection brightness.
In order to realize an appropriate phase difference, the Δnd (refractive index anisotropy×film thickness) may be adjusted appropriately, and in order to enhance the heat resistance and the mechanical strength, a cross-linked structure may be introduced, that is, a compound having at least two polymerizable functional groups may be added appropriately. Meanwhile, as for realization of the high bandwidth reflection wavelength characteristics and the high reflection brightness of the thin film by using the polymerizable cholesteric liquid crystal material, it is possible to realize by using a high Δn polymerizable liquid crystal material and, in addition, using the technique in which compounds having different types of reactivity are combined, as shown in PTL 1.
However, it has been previously difficult to ensure compatibility between the low haze property, the uniformity in the phase difference (orientation state with no variations), and the adhesion. As for the adhesion, PTL 2 describes a method in which an oxime ester based photopolymerization initiator is used, PTL 3 describes a method in which a polymerizable phosphorus based compound is included, and PTL 4 describes a method in which both a silane coupling agent containing an amino group and an alcoholic polyfunctional molecule are added. However, the adhesion on the basis of the technique in PTL 2 or PTL 3 is not sufficient, and the technique in PTL 4 has a problem in that the amount of addition of a non-liquid crystalline compound is too large and, thereby, the transition point (Tni) of the liquid crystal is lowered. For the sake of the uniformity and avoidance of variation phenomenon, it is necessary that the solvent is dried at a temperature lower than or equal to the Tni of the polymerizable liquid crystal composition. However, if the Tni is low, it is not possible to set a sufficiently high drying temperature condition and, thereby, the volatilization rate of the solvent becomes not appropriate and variations occur easily.
Furthermore, enhancement of the solubility into the solvent is also an indispensable condition for the low haze, the uniformity in the phase difference, and avoidance of variation phenomenon. If the solubility into the solvent is poor, the composition becomes nonuniform, the uniformity of the phase difference and the orientation state are lost, and variations occur. Meanwhile, in the case where the polymerizable cholesteric liquid crystal material is used, the planar orientation state is converted to a focal conic state because of nonuniformity in the orientation state, the haze increases significantly by a light scattering effect caused by this focal conic state, and variations occur because the focal conic state is brought about partly.